Boil potatoes until tender. Once cool, remove skin. Mash with food mill or hand mixer. Add milk and mix until incorporated. Don’t over-mix, or potatoes will become glutinous.
Gently mix in salt, pepper, ketchup, mustard, onion, and crabmeat.
Shake out flour on a rimmed plate to prep for shaping croquets.
Combine egg and water for egg wash in small bowl.
Spread panko on second rimmed plate.
Form crab-potato mixture into large potato-shaped croquette (about 3 oz). Should make approximately 8 croquettes.
Roll each croquette through flour, then egg wash, then panko and place on a separate plate.
In a 10-inch sauté pan, add 2 cups canola oil to create a fryer effect and bring to 325–350°F or until a sprinkle of panko sizzles.
Once oil is heated, add croquettes. Brown all sides. Remove and drain on paper towels.
Serve hot next to cabbage slaw and topped with tonkatsu sauce and pickled ginger.